Xylem H-424MS V1.1 - Manual (Page 13)

Chapter 2
SDI-12 Command and Response Protocol
2.1 SDI-12 Command and Response Protocol
Normally the H-424MS operates as a transparent SDI-12 to SDI-12 bridge. However, in order to
wake the remote end of the link from sleep the master H-424MS can be addressed itself as a
“sensor”. The H-424MS comes from the factory with its address set to “0". When the master H-
424MS detects address 0 it responds as a SDI-12 sensor and does not pass the command to the
remote site(s). Issuing the “0M!” command causes the H-424MS to perform a wakeup sequence
to the remote sites. In addition, address 0 allows the Wakeup and Inactivity Timer settings of the
H-424MS link to be observed or modified.
The following is a brief description of the Serial Digital Interface (SDI-12) Command and
Response protocol used by the W
ATER LOG ® Series Model H-424MS “sensor”. Included is a
description of the commands and data format supported by the H-424MS.
Refer to the document "A SERIAL DIGITAL INTERFACE STANDARD FOR HYDROLOGIC
AND ENVIRONMENTAL SENSORS". Version 1.2 April 12, 1996 Coordinated by the SDI-12
Support Group, 135 East Center, Logan, Utah.
During normal communication, the data recorder sends an address together with a command to
the H-424MS. The H-424MS then replies with a "response". In the following descriptions,
SDI-12 commands and responses are enclosed in quotes. The SDI-12 address and the
command/response terminators are defined as follows:
"a" Is the sensor address. The following ASCII Characters are valid addresses:
"0-9", "A-Z", "a-z", "*", "?". The H-424MS will be initially programmed at
the factory with the address of "9". Addresses "0 to 8" and "A to Z" or "a to
z" can be used for additional sensors connected to the same SDI-12 bus.
Address "*" and "?" are "wild card" addresses which select any remote
sensor, regardless of its actual address.
"!" Is the last character of a command block.
"" Are carriage return (0D) hex and line feed (0A) hex characters. They are
the last two characters of a response block.
Notes:
# All commands/responses are upper-case printable ASCII characters.
# Commands must be terminated with a "!" character.
# Responses are terminated with characters.
# The command string must be transmitted in a contiguous block with no gaps of more
than 1.66 milliseconds between characters.
H-424MS
SDI-12 Command and Response Protocol 2-1